The invention relates to an image display of high contrast and a low cost and its manufacturing method.
Since a polysilicon thin film transistor (hereinafter, referred to as TFT) has excellent performance, a circuit can be constructed. As an example which utilizes such a feature, the active-matrix liquid crystal display disclosed in “Society for Information Display International Symposium Digest of Technical Papers”, p. 172, 1999, can be mentioned. According to such a display, by constructing a part of a peripheral driver circuit by the polysilicon TFT, the number of connecting terminals of a pixel portion and the peripheral driver circuit can be reduced, and an image can be displayed at high precision. FIG. 2 shows a constructional diagram of the conventional liquid crystal display using the polysilicon TFT. FIG. 3 shows a plan view of a layout of a pixel 40.
The display of FIG. 2 comprises a gate driver circuit 30, a drain driver circuit 31, and a display area 32. The gate driver circuit 30 and drain driver circuit 31 are constructed by CMOS TFTs 33. In the display area 32, gate-lines 37 and signal-lines 38 are formed in a matrix shape. Each pixel in the display area 32 is constructed by a pixel TFT 34 and a storage capacitor 36 for holding a voltage which is applied to the liquid crystal. Each of those elements has the following functions. First, a voltage is applied to a gate of the pixel TFT 34 from the gate driver circuit 30 and a circuit between a source and a drain is made conductive.
An image signal from the drain driver circuit 31 is supplied to the liquid crystal via the pixel TFT 34, so that the liquid crystal enters a display state corresponding to the signal voltage. The voltage is stored in a liquid crystal capacitor 35 and the storage capacitor 36 until a selective voltage is applied to the gate again at the next period. By providing the storage capacitor 36, a drop of the voltage applied to the liquid crystal due to a leakage current of the TFT can be suppressed and the liquid crystal display having high contrast can be provided. FIG. 4 shows a cross sectional view taken along the line A-A′ in FIG. 3. The liquid crystal capacitor 35 is a capacitor comprising a pixel electrode 13—a liquid crystal layer 21—an opposite electrode 22. By applying a signal voltage to the pixel electrode 13 and changing alignment of the liquid crystal, transmittance of light from a backlight 26 is controlled and a video image is displayed.
According to the conventional liquid crystal display using the polysilicon TFT, since the backlight 26 is irradiated to a polycrystalline silicon layer 4 and a photogenerated carrier is excited, the leakage current increases and the voltage applied to the liquid crystal drops, so that the contrast deteriorates. It is necessary to increase the storage capacitance in order to suppress the drop of the voltage applied to the liquid crystal. However, as shown in FIG. 4, the conventional storage capacitor 36 comprises: a first capacitor which is formed by a polycrystalline silicon layer 14—an insulation film 3 of the same layer as that of a gate insulation film—a storage electrode 8 of the same layer as that of the gate-lines; and a second capacitor which is formed by the storage electrode 8 of the same layer as that of the gate-lines—an interlayer insulation film 10—a passivation film 12—the pixel electrode 13. Since an interval between the electrodes of the second capacitor is 10 or more times as wide as that of the first capacitor, a capacitance of the second capacitor is smaller enough than that of the first capacitor to be ignored. Further, since the interval between the electrodes of the first capacitor is determined by a thickness of gate insulation film, an area has to be increased in order to increase the capacitance. If the area of the storage capacitor is increased, however, an aperture ratio of the pixel decreases and the contrast further deteriorates. The more a pixel area is reduced in order to raise resolution, the more such a problem becomes worse. Therefore, a technique for increasing the capacitance per unit area is necessary.
As a countermeasure against the problem, as disclosed in JP-A-11-271812, there is a method whereby by laminating metal films forming a gate electrode and a storage electrode and the insulation films and patterning them at the same time, insulation films are previously formed onto gate-lines, gate electrode, and storage electrode, and a passivation film and an interlayer insulation film above the storage electrode are etched, thereby forming a storage capacitor comprising the storage electrode, insulation film, and pixel electrode (ITO). According to such a method, the storage capacitance can be increased by thinning the thickness of insulation film constructing the second capacitor. There is, however, a case where the thickness of insulation film forming the capacitor becomes uneven due to an ITO etchant. There is also a case where a reactant layer of the ITO and the insulation film is formed and the storage capacitance decreases.
To prevent the above drawbacks, it is necessary that a barrier metal is formed between the insulation film and the ITO. As disclosed in JP-A-11-271812, there is a method whereby the insulation film and metal film are previously laminated onto the storage electrode, the passivation film and the interlayer insulation film are removed, and thereafter, the pixel electrode is formed. According to such a method, since the metal film plays a role of an etch stopper, the uniform thickness of insulation film under the metal film can be maintained. According to such a method, however, the step of laminating the storage electrode film, insulation films, and metal film and patterning the laminate film and the step of etching only the metal film and the insulation films when a contact hole for connecting the signal-lines and the gate-lines is formed are further added.
According to the method disclosed in JP-A-11-271812, a stairway over which the pixel electrode constructing the storage capacitor rides enlarges, and there is a case where the pixel electrode is disconnected due to such a large stairway.